Common microscopic origin of the phase transitions in Ta2NiS5 and the excitonic insulator candidate Ta2NiSe5

Abstract

The structural phase transition in Ta2NiSe5 has been envisioned as driven by the formation of an excitonic insulating phase. However, the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the phase transition in its complementary material Ta2NiS5 does not show any experimental hints of an excitonic insulating phase. We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta2NiSe5 and Ta2NiS5 using extensive first-principles calculations. In both materials the crystal symmetries are broken by phonon instabilities, which in turn lead to changes in the electronic bandstructure also observed in the experiment. A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap. We conclude that an excitonic instability is not needed to explain the phase transition in both Ta2NiSe5 and Ta2NiS5.

High-Gain MoS2/Ta2NiSe5 Heterojunction Photodetectors with Charge Transfer and Suppressing Dark Current

“…Using DFT simulations, the band structure of pure bulk MoS 2 and Ta 2 NiSe 5 was calculated by the HSE06 functional. As shown in Figure S1, Ta 2 NiSe 5 is a semiconductor with a small band gap of 0.41 eV and MoS 2 is a semiconductor with a band gap of 1.22 eV, which is consistent with previous literature reports. , Then, the charge distribution at the junction is investigated to reveal the charge transfer of the MoS 2 /Ta 2 NiSe 5 heterostructure (see Figure S2 for the computational model). As shown in Figure b, the yellow and cyan regions represent the charge accumulation and depletion, respectively.…”

Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy

“…At the same T C , the crystal symmetry of Ta 2 NiSe 5 changes [33,34], yet without involving periodic lattice distortion nor producing a charge density wave. Even so, recent theoretical [35,36] and experimental [37] studies pointed out that the lattice distortion coupled to the electronic instability is in fact needed to open the band gap. Finally, a recent ARPES study of the VB top of Ta 2 NiSe 5 above T C aimed at the extraction of the entwined photoemission (PE) signature of a precursor state of spontaneously formed excitons [38].…”

Evidence of high-temperature exciton condensation in a two-dimensional semimetal

“…Realization of such a system is difficult because it needs a delicate balance among several factors: the size of the band gap, the binding energy of the electron-hole pairs, and the screening strength 5,7 . Promising candidates so far include TmSe 0.45 Te 0.55 under pressure, Ta 2 NiSe 5 , 1T-TiSe 2 , and other proposed candidates [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] . It is still under intense debate whether the phase transition in Ta 2 NiSe 5 is driven by an exciton condensation or the lattice distortion from a change of the structure 22,23,27,28 , and various competing interpretations have been…”